Sec Form 4 Filing - Raissipour David @ Mimecast Ltd - 2022-05-19

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person Raissipour David 2. Issuer Name and Ticker or Trading Symbol Mimecast Ltd [ MIME] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director _____ 10% Owner X __ Officer (give title below) _____ Other (specify below) Chief Tech. & Product Officer (Last) (First) (Middle) C/O MIMECAST NORTH AMERICA, INC., 191 SPRING STREET 3. Date of Earliest Transaction (MM/DD/YY) 05/19/2022 (Street) LEXINGTON, MA02421 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) __ X __ Form filed by One Reporting Person _____ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Restricted Share Units( 1 ) 05/19/2022 D( 2 )( 3 ) 56,401( 4 ) D 0 D Restricted Share Units( 1 ) 05/19/2022 D( 2 )( 3 ) 13,099( 5 ) D 0 D

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares Share Option (Right to Buy) $ 53.19 05/19/2022 D( 6 )( 7 ) 124,505 ( 6 )( 7 )( 8 ) 07/01/2031 Ordinary Shares 124,505 ( 6 )( 7 ) 0 D

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Raissipour David C/O MIMECAST NORTH AMERICA, INC. 191 SPRING STREET LEXINGTON, MA02421 Chief Tech. & Product Officer Signatures /s/ Robert P. Nault, Attorney-in-Fact 05/23/2022 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Each restricted share unit ("RSU") represents a contingent right to receive one Issuer ordinary share. ( 2 )Pursuant to the terms of that certain Transaction Agreement between Magnesium Bidco Limited ("Magnesium Bidco") and Mimecast Limited ("Issuer"), dated December 7, 2021 (the "Transaction Agreement"), Magnesium Bidco agreed to acquire all of the issued and to be issued ordinary shares of Issuer for $80.00 per ordinary share by means of a court-sanctioned scheme of arrangement (the "Scheme"). At the effective time of the Scheme on May 19, 2022 (the "Effective Time"), each ordinary share then outstanding was automatically transferred from Issuer's shareholders to Magnesium Bidco in accordance with the Scheme and the Transaction Agreement, and Issuer's shareholders ceased to have any rights with respect to the Issuer ordinary shares, except their rights under the Scheme, including their right to receive $80.00 per ordinary share, subject to required withholding taxes. ( 3 )At the Effective Time, each RSU that was vested and outstanding as of immediately prior to the Effective Time ("Vested RSU") was canceled and converted into the right to receive an amount in cash equal to $80.00 multiplied by the aggregate number of shares subject to such Vested RSU, subject to required withholding taxes. Each RSU that was outstanding as of immediately prior to the Effective Time and that was not a Vested RSU ("Unvested RSU") was converted into and substituted with a cash award in an amount equal to $80.00 multiplied by the aggregate number of shares underlying such Unvested RSU immediately prior to the Effective Time, in each case subject to the same time-vesting terms and conditions that applied to the substituted Unvested RSU as in effect immediately prior to the Effective Time, including the requirement of continued service with Magnesium Bidco or its affiliates through the applicable vesting date. ( 4 )These RSUs were granted on July 1, 2021. 25% of the shares vested on July 1, 2022 and the remainder of the shares vest on an annual basis over the next 3 years. ( 5 )These RSUs were granted on December 1, 2021. 25% of the shares vested on December 1, 2022 and the remainder of the shares vest on an annual basis over the next 3 years. ( 6 )Each option to purchase Issuer ordinary shares ("Option") that was vested and outstanding, and unexercised as of immediately prior to the Effective Time ("Vested Option") was canceled and extinguished and converted into the right to receive an amount in cash equal to (x) the difference, if any, between $80.00 and the per-share exercise price of such Vested Option, multiplied by (y) the aggregate number of shares underlying such Vested Option, subject to required withholding taxes. Each Option that was outstanding as of immediately prior to the Effective Time and that was not a Vested Option ("Unvested Option") was automatically converted into and substituted with a cash award in an amount equal to (x) the difference, if any, ( 7 )(Continued from Footnote 6) between $80.00 and the per-share exercise price of such Unvested Option, multiplied by (y) the aggregate number of shares underlying such Unvested Option, in each case subject to the same time-vesting terms and conditions that applied to the substituted Unvested Option as in effect immediately prior to the Effective Time, including the requirement of continued service with Magnesium Bidco or its affiliates through the applicable vesting date. ( 8 )This Option was granted on July 1, 2021. 25% of the shares vested on July 1, 2022 and the remainder of the shares vest on a quarterly basis over the next 3 years.

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